First time posting in this section

I need thoughts and suggestions

My 2014 z51 M7 with 10,000 miles recently developed a noticeable high frequency vibration that I feel in the entire car (not the steering wheel) at speeds above 90 mph. It gets worse the faster I go above 90. The vibration remains when I put the car in neutral and let it glide at that speed. It gradually fades away as the speed drops and then disappears at below 90

My dealer service department has been very kindly working with me along with GM engineering but no problem found yet.

The dealer has twice road force balanced the wheels to no avail. Yesterday he was even kind enough to put a set of brand new wheels and tires from a 2015 z51 on my car and we took car for a road test and it still had the vibration (although it did feel mildly better).

I contacted a performance auto shop and the owner has volunteered to put the car on a dyno machine tomorrow to push it to even higher speeds to see if we can diagnose the problem although he was not very optimistic that the dyno will help

Has any here had any similar problems and if so, any suggestions?

Thanks all

 

Use an electronic vibration analyzer..... Second design....

Essential equipment at all gm dealerships...find one...

Rear half shafts as a guess though....

Rear rotational issues towards hubs...

 

The dealer did bring an analyzer. We used it both on a road test and with the car on a dyno. He hooked the probe of the analyzer under the driver seat. It registered 20-22 hz frequency vibration at 0.7G around 2000-2500 rpm with speeds exceeding 80mph. I am not sure what that all meant but he said that these were frequencies that were out of range (higher) compared to slower speeds and lower rpms readings of the car!

Any thoughts? I really appreciate the help

 

this baffles me.

Ill ask my buddy at our local dealership and see if he may have some insight into it.

 

Mmmm. Have other Vettes had this problem? Why place the probe under the drivers seat? Where exactly was it placed and what was the thinking? How qualified is the technician? I bet if you placed the probe directly onto the chassis, you may get close to 28hz which is the natural frequency of the chassis. That frequency, if it is continuous, suggest that the chassis is been excited by the engine through the mounts. Maybe when the engine slips into 8 cylinder mode or the cam retards or something. You can't trace the energy flow with that probe method. The probe itself becomes an energy sink and then it is guess work to trace the excitation source. Parts replacement to chase the vibration is a crap idea.
Does this vibration occur when the car is stationary and the engine is revved in neutral? What gear were you in? Did you try 5, 6 and 7?
A non contact 3-D scanning vibrometer for data acquisition can make the same measurements as probe type accelerometers but quicker and more accurately and up to third order vibrations can be traced. The advantage of a scanning vibrometer is that sound intensity analysis can be made which is an advantage over sound pressure measurements. Intensity represents the energy flow and reveals more significant flow information than the probe.
Can you hear the noise or just feel the vibration? Engineers want to relate the interior vehicle noise to the forces applied to the body at the engine and chassis mounts and not from wheels and drive train which would give much different orders to what you describe.. Chances are that this vibration is not generated behind the flywheel. Maybe fuel pump. I don't think the alternator or the air pump can excite the chassis to that degree.

 

Following this thread with interest. My guess is either suspension or transmission/diff related. By coasting in neutral (clutch disengaged I assume) you've essentially taken the engine, clutch/PP, and torque tube out of the picture. If the clutch was still engaged but the tranny was in neutral then the speed of the torque tube is unknown but probably still fairly slow.

Good luck, and have a good one,
Mike

 

Oops, missed that detail. 90 mph points to the driveshaft. Vibrations at very high speeds may be due to approaching the drive shaft critical speed or the resonant point where such a vibration can occur. If this occurs, only a new driveshaft or maybe event a redesign will help if other Vettes have this problem. Anything else will show up at lower RPMs. This sort of vibration usually is up in the 8000-10,000 range. Applying torque may shift this resonant point. Flutter the throttle at that point and apply max torque and then coast at that speed and maybe you can detect a vibration RPM shift. If this is the case, this is quite a dangerous vibration and the drive shaft could whip violently. above that speed.

 

But the DS won't be running at speed if the tranny is in neutral; that's why I said it's speed is unknown.

Maybe something having to do with the E-diff?

Have a good one,
Mike

 

When the transmission is placed in neutral on the manual transmission and the clutch engaged, the driveshaft RPM drops to the engine RPM(which will drop to idle RPM). If the manual transmission is in neutral and the clutch is not engaged the driveshaft RPM will drop below engine RPM. When in neutral, the input shaft of the transmission is not connected to the output shaft of the transmission, thus the rear wheels are not driving the driveshaft. On an auto, the driveshaft is always running at engine RPM, would drop to idle(with the foot off the gas pedal) when the transmission is placed in neutral, no matter what the speed of the car is.

The OP stated that when the car was placed in neutral at 90 MPH(and it doesn't matter if his car is a manual or auto) and the car slowly glided down in speed, the vibration also slowly decreased. To me, that means the vibration is speed related(car MPH) thus it is between the rear wheels and the output shaft in the transmission. I would be looking at the transmission, the differential, the half shafts(and CV joints) and the rear wheel bearings. I wouldn't be looking at the driveshaft.

 

What was I thinking?

 

Thanks to all for the hints.
The service manager placed the probe under the seat based on what GM service department aked him to do (because that is where I feel the vibration..ie when I am in the seat!! Where else would I feel it if I am the driver? I KNOW!!!!

No vibration when the engine is reved and the car is stationary.

The vibration stays when the car is in neurtral and gliding above 90mph..but I do not remember if it is the same with the clutch depressed vs the clutch released.

BTW, and I posted this in the general discussion forum..Word I am getting from my dealer is that GM is not obligated to fix this because it is happening at speeds above 70-80 mph!

 

I was waiting for that.

If it did it on the Dyno then that takes the front out of the equation. That leaves everything from the trans to the brakes in play but only the parts within them that move at wheel speed. My money is on a brake rotor.

 

Tell that to the Germans.

 

updates:

The thought now is that it is a "first order vibration" (not sure what that means).

The dealer is going to swap the rotors as the next step and test it to see if additional deeper steps are needed.

 

Lets hope that means they will jump on it and stay on it until it's gone, which I think might be the case.

I wouldn't be happy if mine did what your car is doing, I was very surprised how smooth my car was at 80+ I do hope your car will be the same when they find the bug.

 

I don't know if you would have any luck finding a shop that balances
tires on the car, but this used to be a common practice 30 or 40 years ago. It does have its' advantages because it will include the weight imbalances from drive shafts/brake rotors, etc.

I one time watched a tech (they were called mechanics back then) spin up a wheel with a roller, and then as it coasted down, he would hold his hand on the fender with his index finger held up as a vibration indicator. When his finger quit wiggling, the wheel/tire assembly was in balance. Don't laugh: it worked. The only down side is that you probably would have a hard time balancing the rear on a Corvette.

You can get the rotors balanced. I would take them to somebody who balances fly wheels. Or, just buy new ones and try replacing them, probably one at a time. Rotors are expendables, anyway.

 

Gorden wins. Anyway, this is not the way to do it. You are farting in a thunderstorm. Parts repalcement is chasing the problem instead of isolating it in the first place. In general, order analysis techniques relate sound and vibration signals to a rotating component. These techniques also reduce these signals to characteristic mechanical components and then to where the probe is placed. You can obtain information about individual mechanical parts as well with proper order analysis.
Without a non contact 3D vibrometer, you can replace parts for weeks and still not find the excitement source.
An accelerator type probe can't trace the hierarchy of excitement source orders it can only measure vibration at the probe contact. The vibration you measure at the position you place the probe is not the first order vibration which can be 3 orders or more removed from the excitement source. We have established that we can eliminate the engine and driveshaft. We could have done it sooner had I being paying attention.
If you can't feel it on the steering wheel, we may be able to eliminate the front of the car. The only other rotational components are from the differential pinion shaft to the tires. Tires, wheels, brakes would cause a vibration sooner than 90mph. That leaves axle shafts, bearings, clutch packs (Diff) and the pinion itself. A lot of expensive chasing.
I had trouble with an AWD Chevy Astro van in 91. One hell of a vibration above 60. I went to the dealer the day I took delivery and asked for my money back. He wouldnt give me my money and my lawyer sent him a deadline to pay up after I removed the licence plate and left the car there.
See, this problem did not develop during ownership, I purchased the car with this defect. A big difference that forces them to pay you immediately. They acted quickly as I had purchased 2 Corvettes there. They said that they would give me my money back if they couldn't fix the problem. A dude from GM came with his gear. The front axle and drive shaft was replaced which cured the problem. My lawyer made them extend the warranty for 5 years. I kept the car for two years. The dealer also payed for the lawyer.
If you can prove that the car came from the factory with that defect, they have to give you your money back. My dad bought a Chev and the rear axle broke outside of the guarantee. They charged him for the repair. He took the shaft to his lab where they found a flaw n the forging. Axles normally break at the diff and not at the wheel. My father presented the dealer with the bill which included lab tests and his time. It ended up costing the dealer more than the car was worth after his lawyer got involved. The dealer no longer exists.

 

New rotors apparently made the vibration worse when re-tested on the dyno

Waiting to see what the next step is this week

 

Usually a high speed vibration in these cars is a sign of a drive shaft coupler failing. However, that usually doesn't happen on newer/lower mileage cars. It usually happens on cars driven hard at the drag strip. I pulled the torque tube apart in my 125k mile C5Z06 road race car and the couplers looked brand new vs. drag race buddies who replace them every couple of years.

What happens if you put the car in neutral and disengage the clutch? In this scenario the driveshaft is freewheeling... disconnected from the engine and trans. it should slow down. If that makes the vibration go away faster you know it's the shaft.

 

Doesn't this sound like the problem is out of balance rotors? These rotors are two piece, correct? I think they have a problem with a batch of these rotors. I think you may have found the problem. If they weren't the problem, then the vibration should have stayed the same.